Various circuits, designated window detectors, have been proposed in the prior art which determine whether an input signal is within or outside upper and lower threshold levels. It is often necessary to make this determination relative to a multiplicity of inputs. For example, in air cushion restraint systems it is desirable to monitor the condition of the firing circuit to provide an indication to the operator of any malfunction which might render the firing circuit inoperable in the event of a collision. The firing circuit is a series circuit comprising two normally open acceleration responsive switches which connect the two sides of a plurality of parallel connected detonators between the vehicle battery and ground. Large resistors are connected in parallel with the switches and establish a monitoring current in the series circuit below that required to activate the detonators. The voltage existing at a junction in each detonator circuit is monitored to detect any out of tolerance condition.
Conventionally a two comparator window detector is required for each monitoring point to determine if the voltage being monitored is outside the upper or lower threshold limit of the window detector. When multiple points are to be monitored, multiple window detectors are required with the individual outputs being logically connected to provide a single output. This approach results in complex and costly circuitry.